Ethernet was initially developed for providing network connectivity in a passive shared medium, such as a local area network (LAN). Over time, Ethernet has evolved into an actively switched network that provides bridging and discovers the location of endpoints across the bridged network segments. Where multiple bridges are used to interconnect network segments, multiple potential paths to the same destination often exist. The benefit of this multipath architecture is that it provides path redundancy between bridges and permits capacity to be added to the network in the form of additional links. To prevent loops from being formed, a spanning tree was generally used as the forwarding path for data frames thus restricting the manner in which traffic was broadcast on the network. The basic forwarding principle is to forward everywhere if the destination is unknown and the reachability of destinations is learnt from the source address of data frames; therefore, learning is based on a response to a broadcasted frame. Since both the request and response follow the spanning tree, all of the traffic would follow the links that were part of the spanning tree. This often led to over-utilization of the links that were on the spanning tree and waste for the links that were not part of the spanning tree.
Shortest Path Bridging (SPB) introduces link state routing to Ethernet as a replacement for spanning tree protocols. SPB uses sets of shortest path trees in lieu of a single or a small number of spanning trees. The term SPB covers two modes of operation, SPB-VID (SPBV) mode and SPB-MAC (SPBM) mode, where MAC stands for media access control. The IEEE 802.1aq standard published in 2012 defines a routing solution for Ethernet applicable to PB (IEEE 802.1ad Provider Bridges published in 2005, also known as Q-in-Q) or PBB (IEEE 802.1ah Provider Backbone Bridges published in 2008, also known as MAC-in-MAC). Currently the IEEE 802.1aq standard defines a single routing area for a PB or PBB network.
SPB uses the Intermediate System to Intermediate System (IS-IS) routing protocol. IS-IS is described, for example, in ISO 10589 and IETF RFC 1195, and the extensions for SPB are documented in RFC 6329. IS-IS can be used to synchronize a common repository of information across multiple platforms. It is practical to condense all SPB control and configuration into a single control protocol: the ISIS-SPB protocol. This consolidation is possible because the provider B-MAC, Virtual LAN Identifier (VID) for SPBV, Backbone VID (B-VID) for SPBM and Service Identifier information in the form of I-SID are all global to the network. Connectivity can be constructed using the IS-IS distributed routing system where each node independently computes the forwarding paths and populates the local filtering database (FDB) based on the information in the routing system database.
As the network increases in size, and larger numbers of nodes are included in the network, it may be desirable to divide the network into two or more smaller areas. This allows the control plane to be separated into two or more instances, so that the routing updates may be contained within the smaller routing area and changes within one area do not perturb the adjacent areas. Further, the computational complexity (which tends to be exponential in proportion to network size) benefits from partitioning the network into smaller areas. However, current multi-area networks do not currently embody the concept of multi-pathing as employed by 802.1aq, which is edge based assignment of traffic onto a plurality of Equal Cost Tree sets. As a result, network designs in different areas of the network cannot be easily decoupled from one another.